In general, when a silver halide photographic light-sensitive material is exposed to a light having spectral regions to which the light-sensitive material is sensitive and then developed, an image density increases as an exposure increases, and reaches the maximum value at a certain exposure, and if the exposure further increases, there occurs a phenomenon that the density decreases. This phenomenon is called solarization.
Therefore. where a silver halide emulsion is optically or chemically given an appropriate fog beforehand so that the density reaches the maximum value, the solarization occurs by exposure, whereby a positive image can be directly obtained. A light-sensitive material utilizing such the reversal phenomenon is called a fog nucleus destruction-type direct positive silver halide photographic light-sensitive material (hereinafter referred to as a direct positive light-sensitive material).
Direct positive light-sensitive materials of this type are used for making copies from various photographs.
In recent years, the consumption amount of silver halide photographic light-sensitive materials continues to increase and results in increasing of the processing quantity of light-sensitive materials. To raise the processing efficiency, there has been a strong demand for further more rapid processing.
The above tendency is the case also in the field of X-ray light-sensitive materials; strict enforcement of periodical medical checks leads to rapidly increasing the number of diagnoses, and more accurate diagnoses lead to increase in the number of checking items, which results in more and more increase in a radiographing frequency.
A rapid processing is demanded as well for the direct positive light-sensitive material, and in order to meet this demand, it is necessary not only to automate the diagnoses including radiographing and transport of film but also to provide a rapid processability to the light-sensitive material.
The direct positive light-sensitive material, however, has the problem that the sensitivity is liable to lower when it is subjected to such rapid processing that the overall processing time is 20 to 60 seconds.
The decrease in the sensitivity can be prevented by increasing an amount of coated silver, but it generates such negative effects as increasing of a film production cost and insufficient fixation, washing and drying in processing of film.
Accordingly, in order to provide the rapid processability to the direct positive light-sensitive material. it is necessary to develop a technique for reducing an amount of silver halide without lowering the sensitivity and maximum density. The method for saving silver and maintaining the sensitivity high is described in U.S. Pat. Nos. 2,996,382 and 2,178,382, in which a light-sensitive material having the high sensitivity, contrast and covering power can be prepared by incorporating a surface latent image-type and internal fog-type silver halide grains into the same layer.
The above light-sensitive material, however, has the disadvantage that when it is subjected to a high-speed processing by an automatic processor whose total processing time is from 20 to 120 seconds, a minimum density (fog) is increased and a graininess is deteriorated.
It is known that the minimum density and graininess can be improved by adding various additives to an emulsion or a developer and increasing a gelatin amount, but either is liable to deteriorate the sensitivity, contrast and maximum density.
Most of light-sensitive materials consisting of an insulating support and photographic component layers are liable to accumulate electrostatic charge, which brings about serious problem that static marks are generated on a film and the film is liable to attract dust onto the surface thereof.
The similar problem is caused as well in the direct positive light-sensitive material. Especially in reproducing X-ray photographs, the presence of dust is liable to lead to a wrong diagnosis in medical examinations.
There are a number of conventional antistatic techniques for photographic films for general use as described in Japanese Patent Examined Publication Nos. 8742/1972, 4853/1974, 1617/1981, 19406/1982 and 43729/1983, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 10722/1974, 16525/1975 and 32322/1976.
It has been found that an increased conductivity of a film surface with a nonionic surfactant effectively prevents the film from attracting dust.
However, where the above nonionic surfactane is applied to a direct positive silver halide emulsion produced by methods described in Japanese Patent Examined Publication No. 3938/1975 and Japanese Patent O.P.I. Publication Nos. 43627/1974 and 91632/1974, storage Of the light-sensitive material under a high temperature/moisture condition is liable to cause deterioration of the maximum density thereof.
Generally, there are known two coating methods: one is a continuous coating method in which a silver halide emulsion is continuously prepared and coated: and the other is a batch coating method in which a prescribed quantity of an emulsion is prepared and stored in a storage tank before coating.
The continuous coating method has the advantage that there is no step of storing an emulsion, but continuous coating over a long period of time tends to cause a fluctuation in photographic characteristics in the same lot.
To avoid this problem, it is important to control the adding rates of the additives to the emulsion, which necessitates a very severe process control and maintenance of a complicated and precise control unit and an in-line addition equipment.
On the other hand, the batch coating method, in which a prescribed quantity of an emulsion containing prescribed amounts of additives is prepared and stored while keeping a temperature constant with stirring, has the advantage that there is little fluctuation in photographic characteristics in the same lot to thereby enable to provide uniform quality light-sensitive materials.
The batch coating method, however, causes considerable fluctuation and deterioration of photographic characteristics attributable to storing the emulsion over a long period of time, such as an increase or decrease in the sensitivity and fogging, and such phenomenon becomes conspicuous as the sensitivity of an emulsion increases. which means that the maximum density decreases and the minimum density increases.
There have been reported a number of techniques to use various stabilizers and antifoggants for preventing fluctuation of photographic characteristics attributable to change in the emulsion quality in the course of manufacturing process.
There are known conventional techniques described in Japanese Patent O.P.I. Publication Nos. 217928/1983, 103233/1988 and 61046/1987, but these are insufficient for solving the above problems and providing the light-sensitive materials with super rapid processability.